Perovskites for Photovoltaics

In this chapter, we first model the diffusion length of charge carriers in the active layer of a perovskite solar cell (PSC) of the structure glass/PEDOT: PSS/ CH3NH3PbI3 /PC60BM/Al. It is found that the diffusion length depends on the position x within the active layer measured from the PEDOT: PSS interface, Urbach energy, and temperature. By varying the voltage from 0 to V oc, it is shown that the dependence of diffusion length on the position x in the active layer reduces at higher voltage. The combined influence of the voltage and temperature on the diffusion length of charge carriers is investigated and it is found that in the low voltage range the diffusion length is temperature independent, but it becomes significantly temperature dependent at higher voltages. Also, it is found that the diffusion length decreases as the voltage increases and this reduction becomes much more significant at higher voltage and temperatures. The reduction in the diffusion length due to the increase in Urbach energy becomes less significant at higher voltage. Secondly, using the drift-diffusion model, a new expression for the open-circuit voltage (V oc) in perovskite solar cells is derived. The V oc increases with the ratio of the charge carrier mobilities (μ e /μ h) and by lowering the HOMO energy level of the hole transport layer (HTL). Using the derived V oc, we have found an analytical expression for the bimolecular recombination coefficient, which decreases exponentially with increasing temperature and shows that the weak recombination and high V oc in PSCs is because of the relatively heavy effective mass of polarons. Also, the operating temperature of PSCs under different operating conditions has been calculated. It is found that by reducing the density of tail states at the interfaces through some passivation mechanisms, the operating temperature can be reduced significantly at higher voltages. The results show that if the density of tail states at the interfaces is reduced by three orders of magnitude through some passivation mechanisms, then the active layer may not undergo any phase change up to an ambient temperature of 300 K and it may not degrade up to 320 K.